Binghamton

University

CS-140

Fall 2017

Data Types in Java

1

Binghamton

University

CS-140

Fall 2017

Binghamton

University

CS-211

Fall 2015

Example Class: Car

How Cars are Described

• Make

• Model

• Year

• Color

• Owner

• Location

• Mileage

Actions that can be applied to cars

• Create a new car

• Transfer ownership

• Move to a new location

• Repaint

• Delete a car

2

Binghamton

University

CS-140

Fall 2017

What’s in a class?

• Fields – The data used to describe an object in the class

• Methods – The functions used to manipulate an object in the class

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Binghamton

University

CS-140

Fall 2017

What are Data Types?

• A data type describes how many bits make up the data

• A data type describes how bits should be interpreted

• A data type describes what operations are allowed

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Type Bits Size Encoding Value

int 0000 0000 0000 0000 0000 0000 0000 0110

32

𝑖=0

30

𝑏𝑖 × 2𝑖6

char 0000 0000 0100 0111 16 UTF-16 ‘G’

float 0100 0000 0100 1000 1111 0101 1100 0011

32 −1𝑆 × 1. 𝑠𝑖𝑔 × 2𝑒𝑥𝑝 3.14

Chap 4

Binghamton

University

CS-140

Fall 2017

Java Primitive Data Types

Flavor Type Bits Range Precision

logic boolean ? (8) true or false

exactInteger

byte 8 -128 to +127

short 16 -32,768 to +32,767

char 16 0 to 65,535

int 32 ~-2.14x109 to ~2.14x109

long 64 ~-9.22x1018 to ~9.22x1018

floating pointfloat 32 ~-1038 to ~+1038 ~7 digits

double 64 ~-10308 to +10308 ~15 digits

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Sect. 4.1.1

Binghamton

University

CS-140

Fall 2017

Declaring Local Variables

• Syntax (within a method)

type name;

or

type name=constant;

• Examples

int j;

int count=0;

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count is an instance of an integer whose initial value is zero

Binghamton

University

CS-140

Fall 2017

Declaring Variables

• Variables must be declared and initialized before they are used• Java is an imperative language, and this is typical

• Enables compiler to check to make sure variables are used correctly

• Declaration and initialization can be combined

• Declaration • specifies “type” – how this variable will be used and what values are valid

• specifies “name” – how you will refer to this value from now on

• Initialization• specifies the first value that the variable will have.

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Sect. 2.2

Binghamton

University

CS-140

Fall 2017

Examples of primitive variable declaration

int value = 1; // declaration and initialization

int input; // declaration

input = 5; // initial value or a change of value

char letter = 'g';

char aKoreanChar = '\ud55c'; // 한

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Binghamton

University

CS-140

Fall 2017

More examples:

long large = 1234567890123456789L;

boolean choice = false;

double precipAug = 3.59;

float precipJuly = 2.69F;

byte tiny = (byte)110; // values from -128 to 127

short small = (short) 25000; // from -32768 to 32767

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Binghamton

University

CS-140

Fall 2017

Declaring Fields (or Instance Variables)

• For each data that is used to define an object• We need to define the name of the field• We need to define the type of the field

• Field Declaration Syntax:

class classname {

type fieldname;

type fieldname;

…

}

10

Binghamton

University

CS-140

Fall 2017

Simple example with Primitive Fields

class Account {

long id;

int dollars;

byte cents;

…

}

• Fields used to define what we want to model about an account

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Account

id 7789435

dollars 12417

cents 35

Schematic Account Object

Fields

Class name

Values

Binghamton

University

CS-140

Fall 2017

longitude is a reference to an Angle object

Field Declarations with References

class Angle {

byte degrees;

byte minutes;

double seconds;

…

}

class Position {

Angle latitude;

Angle longitude;

…

}

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seconds is an instance of a double value

Binghamton

University

CS-140

Fall 2017

Field Declarations with References

class Angle {

byte degrees;

byte minutes;

double seconds;

…

}

class Position {

Angle latitude;

Angle longitude; …

}

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Position

latitude

longitude

Angle

degrees 42

minutes 5

seconds 14.78Angle

degrees 75

minutes 58

seconds 5.35

Binghamton

University

CS-140

Fall 2017

Classes as Types

• Primitive types• pre-defined types known to Java

• only 8 : boolean, byte, short, int, long, char, float, double

• Java allows types which are references to objects• “Type” is the class name

• Result is a reference variable… like a pointer in C, but with extra rules

• Reference variables can be thought of as a “handle” on the encapsulated object

14

Binghamton

University

CS-140

Fall 2017

Binghamton

University

CS-211

Fall 2015

Comparing Types and Classes as Types

Data Types

• A data type describes how many bits make up the data

• A data type describes how bits should be interpreted

• A data type describes what operations are allowed

Classes as Types

• A class describes how much data make up the object

• A class describes how to interpret its fields

• A class defines what methods may be applied to an object

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Binghamton

University

CS-140

Fall 2017

Example Field Declarations

class Car {String make;String model;short year;Color color;FullName owner;Position location;double mileage;…

}

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• How Cars are Described• Make

• Model

• Year

• Color

• Owner

• Location

• Mileage

Binghamton

University

CS-140

Fall 2017

Local Reference Variable Examples

• We can declare reference variables:Counter myCounter;

Account myAcc;

String str;

java.io.PrintWriter output;

java.awt.Rectangle rect;

• Reference Variables need to be instantiated before they are used• Instantiation defines which object the reference variable refers to

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Chap. 3

Binghamton

University

CS-140

Fall 2017

Instantiating Reference Variables

• Need to call class constructor to create a new object

myCounter = new Counter();

myAcc = new Account(150.0);

String str= "My String";

//same as: String str = new String("My String");

output = new java.io.PrintWriter(

new java.io.FileOutputStream("out.txt"));

java.awt.Rectangle rect = newjava.awt.Rectangle(10,30,25,60);

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Binghamton

University

CS-140

Fall 2017

Combined Declaration & Instantiation

• Most typically, java code declares a reference variable, and instantiates that reference variable with a new object in the same statement

Counter myCounter = new Counter();

• Notice the required redundancy required• First “Counter” is the type of variable “myCounter”

• Second “Counter” is the invocation of the Counter class creation method

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Binghamton

University

CS-140

Fall 2017

Reference Variables Schematically

Account myAcc = new Account(150.0);

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Account

id 7789436

dollars 150

cents 0myAcc

Binghamton

University

CS-140

Fall 2017

Reference Variables

• Handles on the encapsulated object

• The handle only fits objects in a specific class

• We need the handle to be able to pass the object around and/or use that object

• The handle is either attached to a specific object or not attached to any object (null)

• The handle is attached to an object with an assignment statement

• NO ARITHMETIC ON HANDLES!!!!!

21

Binghamton

University

CS-140

Fall 2017

Instantiating Reference Variables

output = new java.io.PrintWriter(

new java.io.FileOutputStream("out.txt"));

java.awt.Rectangle rect = new java.awt.Rectangle(10,30,25,60);

• To reference the PrintWriter method in the io sub-package of the java package, we need to specify java.io.PrintWriter

• Writing package names every time is a nuisance

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Binghamton

University

CS-140

Fall 2017

Importing Packages

• importing a package allows you to use components without package names

import java.io.*;

import java.awt.Rectangle;

output = new PrintWriter( new FileOutputStream("out.txt"));

Rectangle rect = new Rectangle(10,30,25,60);

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Binghamton

University

CS-140

Fall 2017

Class (Static) Variables

• If you use the “static” keyword, that changes a field into a “class” variable instead of a normal field

• “Class” variables are shared by all objects• Created when the class is loaded – the start of the program

• Deleted when the class is dismissed – the end of the program

• May be initialized when declared

• Often used to keep track of things which a higher level class could take care of

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Binghamton

University

CS-140

Fall 2017

Example Class Variable

class Widget {

String wtype;

String dateMfg;

static string count=0;

public Widget(String t) {

wtype=new String(t);

count++;

… }

public static void howMany() {

System.out.println(

“Built “ + count + “ widgets.”

);

}

… }

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